172 Mr, J. S. Baly on the differential characters 



and in excess of tlie requirements of the species, the super- 

 abundant vital force thus acquired would seem to expend 

 itself in throwing off innumerable varieties, some so dis- 

 similar in habit to the parent form as to be considered by 

 many naturalists specifically distinct. 



Slight alterations in the conditions ; differences of 

 climate, lessened food supply, the inroads of enemies, and 

 other causes too numerous to mention, will check this 

 exuberance of life, and some of the weaker varieties (un- 

 fitted to stand against these changes) will die out, whilst 

 those which remain, in accommodating themselves to 

 their new surroundings, will undergo further modification 

 and eventually attain specific rank. 



Not unfrequently the individuals adapt themselves so 

 entirely to their new state of life that the requirements 

 and means of existence are equally balanced ; and the 

 animals will then continue unchanged for an indefinite 

 length of time, or so long as this balance remains un- 

 disturbed. 



Lastly, the environment of a species is often so adverse 

 to its wellbeing that it gradually succumbs, and, after a 

 more or less prolonged struggle for existence, dwindles 

 away, and finally dies out. 



Every intermediate stage will, as a matter of course, be 

 found between these extremes; but, assuming the doctrine 

 of development to be true, it must be obvious that every 

 animal must live under one or more of the above-named 

 conditions, and it must be equally obvious that the imme- 

 diate surroundings of an animal must determine its per- 

 manence as a species, its development into fresh forms, 

 or its final extinction ; the vital energy of an animal 

 being governed by the favourable or unfavourable con- 

 ditions, or, in other words, the local peculiarities of the 

 spot in which it lives. 



It must happen that a widely-spread species, stretched 

 over an extended district, will be exposed to varying 

 influences, some favourable, others adverse to its neces- 

 sities or requirements of life, and it may be readily con- 

 ceived that each individual, in accommodating itself to its 

 immediate locality, will undergo various modifications, 

 and depart more or less from the parent form. At one 

 point such departure will be but slight, and not extend 

 beyond the limits of a local variety or race ; at another 

 the divergence will be greater, and culminate in the deve- 

 lopment of structural characters, of sufficient inqjortance 



